NASA bets on air-breathing rockets--but what are they?

NASA has put out a fresh call for a little known, but potentially transformative, propulsion technology that could dramatically decrease the cost of launching cargo--and humans--into space.

Issued as an amendment to the "Research Opportunities in Aeronautics 2010" proposal solicitation, this call is the first step in a process that will grant funds to academic institutions and private firms to carry out research on NASA's behalf:

The Airbreathing Access to Space (AAS) system or architecture was chosen to build on work from NASA’s Next Generation Launch Technology (NGLT) Program and other relevant NASA research that enable this entirely new class of airbreathing space access vehicles. It is envisioned that airbreathing propulsion will dramatically increase the reliability and safety of future launch vehicles and ultimately lower the cost of delivering payloads to orbit.

So, what is this mystery technology? The easiest way to think of it is as a next-gen jet engine. In fact, it shares its fundamental characteristics with the engines fastened to the wings of most any commercial aircraft. According to How Stuff Works:

In a conventional rocket engine, a liquid oxidizer and a fuel are pumped into a combustion chamber where they burn to create a high-pressure and high-velocity stream of hot gases. These gases flow through a nozzle that accelerates them further (5,000 to 10,000 mph exit velocities being typical), and then leave the engine.

NASA has determined that it can easily drop the weight of a vehicle at launch if they were to take away the liquid oxidizer...Instead of using liquid oxidizer, an air-breathing rocket, as its name suggests, will take in air from the atmosphere. It will then combine it with the fuel to create combustion and provide thrust.

What differentiates an air-breathing rocket from a typical jet engine is its scope of use. The style of jet engine used in most aircraft ceases to function at speeds in excess of Mach 3 or 4, at which point it gives in to overheating. Jet engines specifically designed for super-high-speed operation tend to be ineffective at lower speeds, or in a launch scenario. What NASA's calling for is an engine capable of working in a heretofore unattainable range of situations:

The development of hypersonic-unique air breathing propulsion systems that operate efficiently and effectively from Mach 0 to 20, and the efficient integration of the airbreathing propulsion system with the airframe are critical to both integrated vehicle performance and controllability. Since these vehicles fly from the Earth’s surface at low speeds and enter space and re-enter the atmosphere at hypersonic speeds, the vehicle performance, controllability, and energy management across the entire Mach range is another significant challenge requiring rapid and accurate computational tools for vehicle design.

The objective is to develop a propulsion system that can help launch a vehicle in two ways: from the ground, and into orbit. Accommodating both scenarios won't be easy, and the low-speed shortcomings of the technology may only be surmountable by supplementing the air-breathing rockets with a more traditional propulsion technology. (Research in the early days of air-breathing rockets seemed to indicate that small, traditional rockets may be necessary for the very first stages of flight, replaced by air-breathing hydrogen rockets after Mach 2 or so.)

The potential payoff for NASA, though, could be massive. For one, the per-pound cost of launching gear and personell into space could be decreased from thousands of dollars to hundreds. Without oxidizers and oxidizer storage, overall craft weight could be dramatically decreased, and vehicle engineers would have much smaller fuel storage units to work around.

The theoretical endpoint of air-breathing rocket research is a reusable, plane-style launch craft that would dramatically lower the cost of space travel, be it government sponsored or private. The rockets needed for such a craft may be years away, but NASA's continued interest in their development is a heartening sign.